132 REPTILIA. 



moult is thus performed, as by Birds, and something analogous 

 occurs not only in many Saurians, but even in Emys among the 

 Chelonia. The exuviation usually takes place in spring, but fre- 

 quently upon the occurrence of a change of weather several times in 

 the course of the year. 



Tegumentary follicles of a particular kind are developed in several 

 Sauria, as the true Lizards, e. g., Lacerta, but also in Iguana, 

 Cordylus, Gecko, though they are here fewer in number. In the 

 above named genera a single row of them provided with from 

 12 to 20 orifices is found extending from the inguinal region to the 

 knee-joint. These apertures are placed upon scales of a particular 

 construction, forming tubercular or bulb-shaped elevations. Each 

 opening conducts into a sac, the commencement of which is divided 

 into small coeca. 



OSSEOUS SYSTEM. 



THE remarkable diversities which the Osseous System of the 

 Reptilia presents throughout the several orders and genera, render its 

 description by no means easy without entering into very consider- 

 able detail. 



As regards the Cranial bones, the best plan will be to consider 

 them separately in each of the orders belonging to the two sub- 

 classes of Reptilia, commencing with the naked Amphibia. The 

 Ichthyodea or Fish-like Amphibia and the Batrachia agree closely 

 in the conditions of their skeleton, though that of the former approxi- 

 mates more closely to Fishes. This is exemplified in the structure 

 of the occipital bone in the Proteus, where the two lateral occipital 

 bones do not articulate by condyles with the vertebral column, but 

 are united firmly by synchondrosis with the first cervical vertebra. 

 In the Salamanders and Frogs, on the contrary, each of the two 

 lateral occipitals has an elongated condyloid process, and both abut 

 against each other superiorly and inferiorly, so as to circumscribe 

 an interjacent foramen magnum. The body of the sphenoid bone 

 is of considerable size, forming the basilar surface of the cranium, 

 and is of a cruciform figure in the Anourous Batrachia, running 

 to a narrow point anteriorly ; it is broad, however, in the Pipa, 

 and to a still greater degree in Siren and in Coecilia. In the 

 direction upward, it supports the alae-majores, which are osseous 

 only in the Tailed Batrachia, being in the Anoura membranous, and 

 perforated by a large opening for the passage of the optic nerve. 

 Each of the inferior wings or alae (processus sphenoidei) is provided 



